Electric vehicles, hybrid electric vehicles (HEV's), plug-in hybrid electric vehicles (PHEV's) and indeed others vehicles that utilize an electric machine, such as an electric motor, may be configured to use the electric machine to provide regenerative braking to at least assist in stopping the vehicle. Regenerative braking may provide a number of advantages over using a friction braking system exclusively. For example, the use of regenerative braking, whereby an electric motor provides negative torque to the vehicle wheels, reduces wear on the friction elements of the friction braking system. In addition, during regenerative braking, the motor may function as a generator, producing electricity that may be used immediately, or stored in a storage device, such as a battery.
Because of the advantages associated with regenerative braking, some regenerative braking control systems may attempt to apply the maximum regenerative braking torque so that overall vehicle efficiencies are maximized. It may be desirable to avoid this strategy, however, if the speed of the motor could go below a minimum desired threshold. This may be of particular importance in a powertrain having a torque-converter clutch, which, when open or slipping, could contribute to the motor speed going below the desired threshold.